Power steering apparatuses are known which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the speed of the electric motor.
Drive control of the electric motor is performed, for example, on the basis of the steering angular speed of the steering wheel. That is, the steering angular speed which corresponds to the time-related change rate of the steering angle of the steering wheel is determined on the basis of an output of a steering angle sensor provided in association with the steering mechanism, and the speed of the electric motor is controlled on the basis of the steering angular speed thus determined.
FIG. 7 is a diagram for explaining the drive control of the electric motor in greater detail, wherein the abscissa and the ordinate represent the steering angular speed V.theta. of the steering wheel and the motor speed R, respectively. Where the steering angular speed V.theta. is not greater than a predetermined first threshold VT1, the motor speed R is kept at the first speed R1. Where the steering angular speed V.theta. is not smaller than a second threshold VT2 which is greater than the first threshold VT1, the motor speed R is kept at a second speed R2 which is greater than the first speed R1. Where the steering angular speed is in a range between the first threshold VT1 and the second threshold VT2, the motor speed R is varied generally linearly with the steering angular speed V.theta. between the first speed R1 and the second speed R2.
Through this control, a greater steering assist force is generated as the steering wheel is more quickly turned and, hence, the steering assist can properly be provided.
When a motor vehicle travels straight, the steering wheel is virtually unoperated. In this case, therefore, virtually no steering assist force is required. In the aforesaid electric motor drive control, however, the electric motor is driven at the first speed R1 even if the steering angular speed V.theta. is at a value close to zero. Accordingly, the electric motor is kept driven even though no steering assist is required. This makes impossible to avoid wasteful energy consumption.